Field of the Invention
The present invention relates to a biosensor and a method of quantitative analysis using the same. More particularly, the present invention relates to a biosensor and a method for detecting and quantitatively analyzing a specific material contained in a liquid sample such as blood, serum, urine, saliva and the like.
Examples of the specific material to be assayed are glucose, lactic acid, cholesterol, alcohol, urea and the like. Examples of compounds which specifically react with these compounds are oxidoreductases. Other electrochemically active compounds include ascorbic acid, uric acid and the like.
While there are various specific materials to be assayed as above, a conventional assay will be explained by making reference to quantitative analysis of lactic acid and ascorbic acid as an example.
Lactic acid reacts with lactate oxidase to produce an electrochemically active material. By measuring an electric current generated in this reaction, a concentration of lactic acid in the liquid sample can be measured. However, the liquid sample often contains other electrochemically active compounds such as ascorbic acid, which interferes an accurate measurement of the concentration of lactic acid.
To solve such problem, the applicants proposed a method of quantitative analysis using a biosensor which comprises a working electrode and a counter electrode and in which a biologically active material such as an enzyme is placed at a part which is remote from an electrode surface so as to separately analyze a desired compound and ascorbic acid (see U.S. patent application Ser. No. 07/883,367 filed on May 15, 1992 and European Patent Application No. 92 108 197.2 filed on May 15, 1992). In this biosensor since the enzyme is placed at a part remote from the electrode, in an early period from the supply of the sample, an electric signal which depends solely on ascorbic acid is detected and thereafter a reaction product of an enzymatic reaction diffuses and reaches the electrode surface whereby a sum of the electric signal depending on ascorbic acid and that depending on the reaction product is detected. Then, the electric signal depending on the reaction product is calculated from the detected signal values.
In the above biosensor, the separation of the electric signals relies on the diffusion of the reaction product. When the blood is analyzed, blood corpuscles greatly influence the diffusion rate so that a hematocrit may have an influence on sensitivity of the assay. When the blood containing a large amount of particles such as erythrocytes is assayed, it is necessary to carry out the assay on the separated serum, while there is no problem when urine or saliva is assayed.